The present invention generally relates to cathodes for solid oxide fuel cells (SOFCs) and, more particularly, to a multi-layered, multifunctional cathode having high conductivity, high catalytic activity, minimized coefficient of thermal expansion (CTE) mismatch, excellent compatibility to other portions of the fuel cell, and reduced temperature operation.
A solid oxide fuel cell is an energy conversion device that produces direct-current electricity by electrochemically reacting a gaseous fuel (e.g., hydrogen) with an oxidant (e.g., oxygen) across an oxide electrolyte. The key features of current SOFC technology include all solid-state construction, multi-fuel capability, and high-temperature operation. Because of these features, the SOFC has the potential to be a high-performance, clean and efficient power source and has been under development for a variety of power generation applications.
Under typical operating conditions, an SOFC single cell produces less than 1V. Thus, for practical applications, single cells are stacked in electrical series to build voltage. Stacking is provided by a component, referred to as an interconnect, that electrically connects the anode of one cell to the cathode of the next cell in a stack. Conventional SOFCs are operated at about 1000xc2x0 C. and ambient pressure.
A SOFC single cell is a ceramic tri-layer consisting of an oxide electrolyte sandwiched between an anode and a cathode. The conventional SOFC materials are yttria-stabilized zirconia (YSZ) for the electrolyte, strontium-doped doped lanthanum manganite (LSM) for the cathode, nickel/YSZ for the anode, and doped lanthanum chromite for the interconnect. Currently, there are two basic cell constructions for SOFCs: electrolyte-supported and electrode-supported.
In an electrolyte-supported cell, the electrolyte is the mechanical support structure of the cell, with a thickness typically between 150 and 250 xcexcm. Electrolyte-supported cells are used, for example, in certain planar SOFC designs. In an electrode-supported cell, one of the electrodes (i.e., the anode or cathode) is the support structure. The electrolyte is a thin film (not greater than 50 xcexcm) that is formed on the support electrode. Tubular, segmented-cells-in-electrical-series, and certain planar SOFC designs, employ this type of cell.
Conventional YSZ-based SOFCs typically employ electrolytes thicker than 50 xcexcm and require an operating temperature of 1000xc2x0 C. to minimize electrolyte ohmic losses. The high-temperature operation imposes stringent material and processing requirements to the fuel cell system. Thus, the recent trend in the development of SOFCs is to reduce the operating temperature below 800xc2x0 C. The advantages of reduced temperature operation for the SOFC include a wider choice of materials, longer cell life, reduced thermal stress, improved reliability, and potentially reduced fuel cell cost. Another important advantage of reduced temperature operation is the possibility of using low-cost metals for the interconnect.
Data and information in the literature indicate that SOFC cells can be further developed and optimized to achieve high power densities and high performance at reduced temperature. The electrolyte and cathode have been identified as barriers to achieving efficiency at reduced operating temperatures due to their significant performance losses in current cell materials and configurations.
Various attempts have been made to reduce the operating temperature of YSZ-based SOFCs while maintaining operating efficiency. With YSZ electrolyte-supported cells, the conductivity of YSZ requires an operating temperature of about 1000xc2x0 C. for efficient operation. For example, at about 1000xc2x0 C. for an YSZ electrolyte thickness of about 150 xcexcm and about a 1 cm2 area, the resistance of the electrolyte is about 0.15 ohm based on a conductivity of about 0.1 S/cm. The area-specific resistance (ASR) of the electrolyte is, therefore, about 0.15 ohm-cm2. For efficient operation, a high-performance cell with an ASR of about 0.05 ohm-cm2 is desired. To achieve an ASR of about 0.05 ohm-cm2 at reduced temperature operation (for example, 800xc2x0 C.), the required thickness (i.e., 15 xcexcm) of YSZ can be calculated. If the desired operating temperature is less than 800xc2x0 C., while the ASR remains the same, either the thickness of YSZ must be further reduced or highly conductive alternate electrolyte materials must be used.
Various methods have been evaluated for making cells with thin films (about 5 to 25 xcexcm thick). Electrode-supported cells (specifically, anode-supported cells) with thin electrolyte films have been shown high performance at reduced temperatures. Power densities over 1 W/cm2 at 800xc2x0 C. have been reported, for example, in de Souza et al., YSZ-Thin-Film Electrolyte for Low-Temperature Solid Oxide Fuel Cell, Proc. 2nd Euro. SOFC Forum, 2, 677-685 (1996); de Souza et al., Thin-film solid oxide fuel cell with high performance at low-temperature, Solid State Ionics, 98, 57-61 (1997); Kim et al., Polarization Effects in Intermediate Temperature, Anode-Supported Solid Oxide Fuel Cells, J. Electrochem. Soc., 146 (1), 69-78 (1999); Minh, Development of Thin-Film Solid Oxide Fuel Cells for Power-Generation Applications, Proc. 4th Int""l Symp. On SOFCs, 138-145 (1995); Minh et al., High-performance reduced-temperature SOFC technology, Int""l Newsletter Fuel Cell Bulletin, No. 6, 9-11 (1999). An alternative attempt at reducing operating temperature has involved the use of alternate solid electrolyte materials with ionic conductivity higher than YSZ, as described in Minh, Ceramic Fuel Cells, J. Am. Ceram. Soc., 76 [3], 563-88 (1993). However, the work on alternate electrolyte materials is still at a very early stage.
The other barrier to achieve efficiency at reduced temperature is the cathode 13. LSM-based cathodes have been used in high-temperature ( greater than 900xc2x0 C.) SOFCs as either a porous structure of sintered LSM particles or as LSM/YSZ mixtures. For operation at reduced temperatures (e.g., 700 to 900xc2x0 C.), optimization of the mixtures of LSM and YSZ in the cathode has resulted in a cathode ASR of 0.2 to 0.3 ohm-cm2 at 800xc2x0 C. For thin-film electrode-supported cells, the total cell ASR is typically less than 0.4 ohm-cm2. The performance and losses from each of the cell components of a typical thin-film YSZ electrolyte with an Ni/YSZ anode-support electrode and with an optimized LSM/YSZ cathode are showed in FIG. 1. As seen in the figure, the loss from the cathode contributes to the majority of the total cell performance loss. When the cell operating temperature is decreased, the cell ASR increases significantly due to an increase in both electrolyte resistance and cathode polarization.
Recently, there have been attempts to increase performance at reduced operating temperatures by developing new cathode materials in combination with new and higher-conductivity electrolytes. These cathode materials are typically designed to overcome the limitations from LSM""s oxide ion conductivity described in Steele, Survey of Materials Selection for Ceramic Fuel Cells II. Cathode and Anode, Solid State Ionics, 86-88, p. 1223 (1996), the rate of oxygen exchange reaction on the LSM surface, and the moderate electronic conductivity of LSM. One approach involves the development of Ag/yttria-doped bismuth oxide (YDB) cermet cathodes for doped ceria (CeO2) electrolytes. The combination of high oxide ion conductivity of YDB and high electronic conductivity of Ag yielded some enhancement in performance between 500 and 700xc2x0 C. as discussed in Doshi et. al., Development of Solid-Oxide Fuel Cells That Operate at 500xc2x0 C., J. Electrochem. Soc., 146 (4), 1273-1278 (1999), and Wang et. al., Lowering the Air-Electrode Interfacial Resistance in Medium-Temperature Solid Oxide Fuel Cells, J. Electrochem. Soc., 139 (10), L8 (1992). However, YDB is not suitable for use above 700xc2x0 C. as it reacts with ceria. In addition, Ag tends to densify above 700xc2x0 C., thus decreasing porosity for gas access.
Another approach has involved the development of a single material with the combination of desirable properties mentioned above. For example, some materials in the Laxe2x80x94Srxe2x80x94Fexe2x80x94Coxe2x80x94O system, such as La0.6Sr0.4Fe0.8Co0.2O3 (LSCF) and La0.6Sr0.4CoO3 (LSC), possess much higher ionic and electronic conductivity compared to LSM. Preliminary data on the use of such materials with ceria and (Sr- and Mg-doped LaGaO3) LSGM electrolytes, which is published in Doshi et. al., Development of Solid-Oxide Fuel Cells That Operate at 500xc2x0 C., J. Electrochem. Soc., 146 (4), 1273-1278 (1999), and Steele et. al., Properties of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) double layer cathodes on gadolinium-doped cerium oxide (CGO) electrolytes II. Role of oxygen exchange and diffusion, Solid St. Ionics, 106, 255 (1998), show some improvement.
Attempts to dope a LSC cathode with a small amount of Ni resulted in a cell peak power density of 400 mw/cm2 at 650xc2x0 C. as described in Visco et. al., Fabrication and Performance of Thin-Film SOFCs, Proc. 5th Int""l Symposium on SOFCs, 710 (1997), implying a cathode ASR of no more than 0.4 ohm-cm2 at 650xc2x0 C. The limitations of these materials include high reactivity with YSZ at temperatures above 800xc2x0 C. (discussed in Kindermann et. al., Chemical Interactions between Laxe2x80x94Srxe2x80x94Mnxe2x80x94Fexe2x80x94Oxe2x80x94Based Perovskites and Yttria-Stabilized Zirconia, J. Am. Ceram. Soc., 80[4]909-914 (1997)) and high CTE (depending on Co content) compared to available electrolytes. LSC has a CTE of almost 23xc3x9710xe2x88x926 in./in./xc2x0 C. and LSCF has a CTE of about 14xc3x9710xe2x88x926 in./in./xc2x0 C.
Other efforts involve development of low-temperature electrodes for oxygen generation. However, many of those materials may not be suitable for SOFC applications. For example, materials termed BICUVOX, which are made from the Bixe2x80x94Cuxe2x80x94Vxe2x80x94O family, have high oxygen conductivity in certain directions of the molecular structure but are highly reactive and less stable than desired for SOFC applications.
The fabrication process that the above materials undergo is an important factor that affects the performance of a fuel cell. Several techniques are available to manufacture cells in either of the two classes of cell construction (i.e. electrolyte-supported and electrode-supported), including thick-film electrolytes and thin-film electrolytes.
For thick-film electrolytes, tape casting is typically used to fabricate these dense membranes. During tape casting, a slurry of fine ceramic particles dispersed in a fluid vehicle is cast as a thin tape on a carrier substrate using a doctor blade. The tape is then dried, removed from the carrier substrate, and fired to produce a dense substrate. After sintering, deposition techniques such as hand painting, screen-printing, or spray coating are used to attach electrodes to both sides. The high ohmic resistance of the thick electrolyte necessitates higher operating temperatures of around 1000xc2x0 C. to reduce the ohmic polarization losses due to the electrolyte.
Driven by the benefits of reducing ohmic loss in the electrolyte at lower temperatures (i.e., 550 to 800xc2x0 C.), SOFC development efforts have focused attention on xe2x80x9cthin-film electrolytesxe2x80x9d (i.e., 5 to 25 xcexcm) supported on thick electrodes, such as described in U.S. Pat. No. 5,741,406. A number of selected fabrication processes used for making SOFCs, especially thin YSZ electrolytes, are listed in Table 1.
Other thin-film techniques investigated for SOFC applications include vapor-phase electrolytic deposition, vacuum evaporation, liquid-injection plasma spraying, laser spraying, jet vapor deposition, transfer printing, coat mix process, sedimentation method, electrostatic spray pyrolysis, and plasma metal organic chemical vapor deposition. Additional related references are found in U.S. Pat. Nos. 5,922,486; 5,712,055; and 5,306,411.
As can be seen, there is a need for an SOFC fabrication process that ensures that no condition or environment in any process step destroys the desired characteristics of any of the materials. Another need is for electrode properties which provide increased performance in the 550 to 800xc2x0 C. range while maintaining function integrity up to 1000xc2x0 C. for short periods. One goal is to achieve a cell power density of about 1 W/cm2 at about 600xc2x0 C., as well as a cathode ASR that is less than 0.35 ohm-cm2 at such temperature. Based on the performance of current optimized LSM/YSZ cathodes ( greater than 1.5 ohm-cm2 at 600xc2x0 C.), a significant increase in performance is required. New cathodes with improved properties must be employed in conjunction with the electrolyte to create a compatible system of fuel cell components. Some of the improvements should include increased catalytic activity for oxygen reduction reaction, increased ionic conductivity near the interface, and electronic conductivity at the electrode surface.
The cathode of the present invention provides a high-performance, reduced-temperature SOFC. The cathode is based on materials and structures which, when combined, are capable of increased performance in about the 550xc2x0 to 800xc2x0 C. operating range while maintaining functional integrity up to about 1000xc2x0 C. The materials and fabrication processes are economical, scalable, and amenable to high-volume manufacture of fuel cells.
The cathode of the present invention for SOFCs is preferably multi-layered and multifunctional, having high conductivity (about 100 to 5000 S/cm), high catalytic activity, minimized coefficient of thermal expansion (CTE) mismatch, excellent compatibility to other portions of the fuel cell, such as electrolyte and interconnect, and can operate at reduced temperatures. The cathode will allow efficient operation at temperatures between 550 xc2x0 to 800xc2x0 C. rather than the conventional 1000xc2x0 C. The low operating temperature range will enable material selections that are more economical and possess desired characteristics.
In one aspect of the present invention, a solid oxide fuel cell comprises an anode, an electrolyte adjacent to the anode, and a cathode adjacent to the electrolyte, with the cathode having a conductive layer adjacent the electrolyte.
In a further aspect of the present invention, a cathode in a solid oxide fuel cell comprises a conductive layer having a first density, a catalyst layer having a second density that is less than the first density, and a graded composition layer characterized by a graded electronic conductivity and a graded ionic conductivity.
In another aspect of the present invention, a method of making a cathode for a solid oxide fuel cell comprises producing a conductive layer having a first density and producing a catalyst layer having a second density that is less than the first density, with the catalyst layer being adjacent the conductive layer.
Yet in another aspect of the present invention, a method of making a cathode for a solid oxide fuel cell further comprises producing a graded electronic conductivity in a graded composition layer adjacent a catalyst layer and producing a graded ionic conductivity in the graded composition layer adjacent the catalyst layer.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.